Method of controlling the supply of al{11 o{11 {0 during the operation of a cell for electrolytic recovery of aluminum

ABSTRACT

Method of controlling the supply of Al2O3 during the operation of a cell for electrolytic recovery of aluminum in a fluoride melt.

United States Patent [191 Schmidt-Hatting et al.

[111 3,850,768 Nov. 26, 1974 METHOD OF CONTROLLING THE SUPPLY OF AL O DURING THE OPERATION OF A CELL FOR ELECTROLYTIC RECOVERY OF ALUMINUM [75] Inventors: Wolfgang Schmidt-Batting, Chippis;

Kiranendu Chaudhuri, Gampel; Peter Bachofner, Liebefeld, all of Switzerland [73] Assignee: Swiss Aluminium Ltd., Chippis,

Switzerland [22] Filed: July 10, 1973 211 Appl. No.: 378,233

[30] Foreign Application Priority Data Switzerland 10750/72 July 18, I972 [52] US. Cl. .I 204/67 [51] Int. Cl C22d 3/12 [58] Field of Search 204/67 [56] References Cited UNITED STATES PATENTS 3,583,896 6/1971 Piller 204/67 3,712,857 1/1973 Filler 204/67 Primary Examiner-John H. Mack Assistant Examiner-D. R. Valentine Attorney, Agent, or Firm-Ernest F. Marmorek [57] ABSTRACT Method of controlling the supply of A1 0 during the operation of a cell for electrolytic recovery of aluminum in a fluoride melt.

2 Claims, 2 Drawing [Figures METHOD OF CONTROLLING THE SUPPLY OF AL O DURING THE OPERATION OF A CELLFOR ELECTROLYTIC RECOVERY OF ALUMINUM This method is based on the indirect determination of the concentration of A1 in the melt, by measuring of the fraction E of the EMF, independent of current density but related to the said concentration. This measuring is carried out by extrapolating to the cell voltage at cell current equalling zero from the values of voltage and current intensity in a filed of straightness of the voltage-current graph.

For the recovery of aluminum by electrolysis of aluminum oxide (A1 0 alumina) the latter is dissolved in a fluoride melt, which consists in the greatest part of cryolite Na AlF This melt is contained in a cell having a carbon bottom. The aluminum separated at the cathode collects in liquid state on the carbon bottom of the cell beneath the fluoride melt, and the upper surface of this-liquid aluminum in fact constitutes the cathode. Anodes of amorphous carbon dip from above into the melt. Oxygen arises at the anodes by the electrolytic decomposition of the aluminum oxide, and combines with the carbon of the anodes to CO and C0 The electrolysis takes place in a temperature range of about 940 to 975C.

The principle of an aluminum electrolysis cell with prebaked anodes appears from FIG. 1 of the accompanying drawing. This shows a vertical section in the longitudinal direction through part of a known electrolysis cell.

The steel shell 12, which is lined with a thermal insulation 13 of heat-resisting, heat-insulating material and withcarbon 11, contains the fluoride melt (the electrolyte). The aluminum 14 separated at the cathode lies on the carbon bottom 15 of the cell. The surface 16 of the liquid aluminum constitutes the cathode. In the carbon lining 11' there are inserted iron cathode bars 17 transverse to the longitudinal direction of the cell, and these conduct the electrical direct current from the carbon lining ll of the cell laterally outwards. Anodes 18 of amorphous carbon dip from above into the fluoride melt l0, and supply the direct current to the electrolyte. They are firmly connected via conductor rods 19 and clamps 20with the anode beam 21. The current flows from the cathode bars 17 of one cell to the anode beam 21 of the following cell through conventional current bus bars, not shown. From the anode beam 21 it flows through the conductor rods '19, the anodes 18, the electrolyte 10, the liquid aluminum 14, and the carbon lining 11 to the cathode bars 17. The electrolyte 10 is covered with a crust 22 of solidified melt and there is a layer of aluminum oxide 23 lying above the crust. In operation, cavities 25 occur between the electrolyte l0 and the solidified crust 22. Against the side walls of the carbon lining 11 there likewise forms a crust of solid electrolyte, namely a lateral ledge 24. The horizontal extent of the lateral ledge 24 affects the plan area of the bath of liquid aluminum l4 and electrolyte 10.

The distance d from the lower side 26 of the anode to the surface 16 of the liquid aluminum, also known as the interpolar distance, can be adjusted by lifting or lowering of the anode beam 21 with the help of the lifting mechanism 27, which is mounted on pillars 28. This effects all the anodes. An anode can be adjusted individually by releasing the respective clamp 20, shifting the respective conductor rod 19 upwards or downwards relatively to the anode beam 21, and retightening the clamp.

Because of attack by the oxygen released during electrolysis, the anodes are consumed continuously on their lower side, by about 1.5 to 2 cms per day according to the type of cell. At the same time, the height of the liquid aluminum on the bottom of the cell increases continuously-by about 1.5 to 2 cms per day due to the aluminum separated at the cathode.

When an anode has been consumed, then it is exchanged for a fresh anode. In practice, the cell is operated in such a way that, some days after its start of use, the anodes of the cell no longer have the same degree of consumption, and therefore. they must be exchanged separately over a range of several weeks. For this reason, anodes of different starting dates operate together in the same cell, as appears from the drawing.

Because of this complex situation, the interpolar distances a of individual anodes are not exactly equal to each other. It suffices for the purpose of the present invention to consider the average, at any moment in time, of the individual interpolar distances. This average interpolar distance, which itself varies with time, will be termed D.

The principle of an aluminum electrolysis cell with one or more self-baking anodes (Soederberg anodes) is the same as that of an aluminum electrolysis cell with pre-baked anodes. Instead of pre-baked anodes, one or more anodes are used which are continually baked from a green electrode paste in a steel jacket during the electrolytic operationby the heat of the cell. The direct current is supplied by lateral steel rods or from above by vertical steel studs. These anodes are renewed as required by pouring green electrode paste into the steel jacket. Adjustments of interpolar distance are made by vertical adjustments of the steel jacket.

By breaking in of the upper electrolyte crust 22 (the crusted bath surface), the aluminum oxide 23 which is above it is brought into the electrolyte 10. This operation is known as servicing of the cell. In the course of the electrolysis, the electrolyte becomes depleted in aluminum oxide. When the concentration of aluminum oxide in the electrolyte falls'to somewhere between 1 and 2 percent, there arises the anode effect, which re I The aluminum 14 produced electrolytically, which collects on the carbon bottom 15 of the cell, is generally removed once a day from the cell by conventional tapping devices, for instance sucking devices.

One measureable quantity in the operation of the cell is its base voltage. This depends on the age of the cell, the condition of the carbon lining 11, and the composition of the molten electrolyte 10, as well as on the cell current intensity and current density. The base voltage is also affected by the variation of the plan area of the bath in consequence of variation of the horizontal extent of the lateral ledge 24. The base voltage is measured between corresponding points on the anode beams of the cell in question and of the next cell in series. The voltage is the total of the ohmic voltage drops in the parts of the cell through which current flowsplus the EMF required for the electrolytic decomposition of the M in the electrolyte.

There is an optimum value of the base voltage, and this corresponds to an optimum average interpolar distance D. In practice the actual average interpolar distance is sometimes larger or smaller than the optimum average interpolar distance. The departures are substantially produced by increase of the height of the liquid aluminum 14 above the carbon bottom 15, and by burning away of the anodes 18 at their lower side 26, as well as variations of the thickness of the lateral ledges of frozen electrolyte.

The base voltage of a cell can be expressed in the following formula:

In this formula:

U signifies base voltage (volts) E EMF (volts) 2 IR total of all ohmic voltage drops (volts) The EMF generally is combined of a term E independent of current density and a term E, dependent on current density. One can obtain E, from a voltagecurrent graph by extrapolation to cell Current equal to zero. E is substantially dependent on the Al O concentration of the fluoride melt 10.

Now it is valuable to monitor the M 0 concentration of the fluoride melt, because this enables one to adjust the supply of A1 0 at the normal servicings of the cell, so as to avoid too many anode effects due to too low concentration and also to avoid deposits on the bottom of the cell due to too high concentration. The A1 0 concentration of the fluoride melt can be determined directly or indirectly.

The direct method involves taking a sample and its direct chemical analysis. This requires a lot of time and is not applicable during cell operation.

The indirect method involves determination of the term E independent of current in the EMF. This can lead to great errors. For extrapolation, use has hitherto been made of 'the current intensity and voltage at the normal working'point (i.e., at the point on a voltagecurrent graph representing normal working conditions that is to say representing nominal current intensity and nominal voltage), and at a second point with reduced current intensity. As has been found in the course of extensive researches by the inventors, the voltagecurrent graph in the neighbourhood of the normal working point frequently does not run straight. This is the reason why from extrapolation one usually does not obtain the correct value E This disadvantage of the indirect method is avoided by use of methods according to the invention.

According to this invention a method of controlling the supply of A1 0 during the operation of a cell for electrolytic recovery of aluminum, comprises the following operations carried out successively: (a) measuring the cell current intensity at a normal working point: (b) making a first reduction of the current intensity in one or more steps until attainment of cell stability and of straightness of the voltage-current graph: (c) measuring the values of voltage and current intensity of the cell at a time between half a minute and 2 minutes after the first current intensity reduction: (d) making a second reduction of current intensity to a total current intensity reduction of at most 25 percent, but at least of percent more than the first reduction, up to at latest 10 minutes after the first current intensity reduction, the percentages being of the current intensity at the working point: (e) measuring the values of voltage and current strength of the cell at a time between half a minute and 2 minutes after the second current intensity reduction: (f) restoring the current intensity to the normal working point: (g) extrapolating to the cell voltage at cell current equalling zero from the values of voltage and current intensity after the first current intensity reduction and after the second current intensity reduction: (h) determiningthe concentration of A1 0 in the cell on the basis of the known relationship between the term E of the EMF independent of current density and the A1 0 concentration: (i) supplying a larger or smaller addition of A1 0 to the electrolyte at each servicing of the cell, if the A1 0 concentration departs downwards or upwards from a predetermined value. It is well known that current efficiency depends on the instantaneous A1 0 concentration. The appropriate predetermined value for A1 0 concentration is, for the skilled art worker, the value that gives optimum cell current efficiency.

The researches of the inventors have brought to light that the voltage-currcnt graph in most cases begins to run straight after around a 5 percent reduction of current intensity from the working point. At any rate this value of 5 percent can be checked by a simple test for each cell. It is to be recommended, not to go over about 8 percent at the first current intensity reduction, because at the second current intensity reduction a total reduction of 25 percent, reckoned from the normal working point, should not be exceeded, as is explained further below. If in fact the value of 8 percent is exceeded at the first current intensity reduction, then the differences in the values after the second current intensity reduction may be too small for accuracy, and thus too great errors will arise in the result of extrapolation.

A' time intervalbetween' thefir'st' 'ar'i'd'the' second egrrent intensity reduction is necessary, so that one can confirm that, after the first current intensity reduction, cell stability hasarisen (no significant fluctuations in voltage with constant cell current). On the other hand this interval must not exceed 10 minutes. This limit is to avoid the temperature of the electrolyte decreasing too much so that its specific electrical conductivity alters.

It may be that, after the first current intensity reduc: tion, cell stability is not reached. A cell always has I small voltage fluctuations, even if the current intensity is constant. These can be measured at the normal working point. Instability is present if the fluctuations become larger than normal. This may be caused by magnetic effects. If cell stability is not reached after the first current intensity reduction, then the current intensity must be reduced further by a slight percentage step. This further step forms a further part of the first current intensity reduction." The total of the steps must not exceed 8 percent, or the whole procedure must be broken off and repeated at a later point in time. The first current intensity reduction can thus take place for example in two steps, a first step of 5 percent and a second of 2 percent, in total 7 percent in the first current intensity reduction.

The second current intensity reduction must, in relation to the first current intensity reduction. produce an absolute difference of at least l0 percent, so that, as explained more fully above, a sufficiently large difference for extrapolation is achieved between the voltage values. On the other hand, the total of the first and second current intensity reductions should not exceed 25 percent reckoned from the normal working point, because cell current intensities which are too low so strongly influence the magnetic conditions in the liquid aluminum that the cell voltage is falsified, for example by alteration of the interpolar distance.

The limitations of about half a minute are necessary, because there is a certain time lag with which the voltage follows a change of current intensity. The limitations of 2 minutes are necessary because decrease of temperature will arise very quickly due to the lower current intensity, especially after the second current reduction, and a lower temperature corresponds to a lower electrical conductivity.

In a plant comprising numerous cells in series, the measurements preferably take place centrally for all cells through a computer. The cell current intensity is the same for all the cells and so may be measured at a single place, with the help for example of a direct current transformer. Such a direct current transformer is usually incorporated in the rectifier system.

The voltage of each cell can be measured between corresponding points on the anode beams 21 (FIG. 1) of two successive cells. These voltages are supplied to the computer via suitable electric conductors.

FIG. 2 displays the opeations according to the invention in a diagram. What is shown is the cell current intensity I in amps as a function of time t in seconds.

Between 50 and 51 the cell current intensity is shown at the normal working point. Here a measurement is made. The cell current intensity amounts for example to 100,000 amps. At 51 occurs the first current intensity reduction to a lower value 52, eg to 95,000 amps which corresponds to a current intensity reduction of 5 percent (5,000 amps absolute). Between 52 and 53, voltages and current intensity are sampled from all the cells to be measured, and the measured values obtained are checked for cell stability and forlinearity'of voltage-current relationship. Linearity is detected by the slope of the voltage-current graph being constant.

If stability and linearity are found between 52 and 53, then the second current intensity reduction occurs at 53 down to 56, for example through a further percent decrease 15,000 amps absolute) to 80,000 amps. Between 56 and 58, voltages and current intensity are again sampled from each of the cells to be measured. From the values obtained on the one hand between the points 52 and 53 and on the other hand between the points 56 and 58, the term E of the EMF independent of current density is calculated for each cell by extrapolation. At 58 the measurements are ended, and the normal current intensity e.g., 100,000 amps, can be again established, which is achieved at the point 59.

If there is a lack of stability in any of the cells between 52 and 53, a further step of the first current intensity reduction can be undertaken to 54, e.g., to 92,000 amps (which corresponds to a further current intensity reduction of 3 percent), and a check again be made between 54 and 55 for cell stability. if cell stability is achieved, the second current intensity reduction of 12 percent to 57 (80,000 amps) occurs at 55 to 57. Voltages and current intensity are sampled from each of the cells between 57 and 58. If there is lack of stability between 54 and 55, one goes back to the normal starts the operations all over again after a time lapse of, say, one hour, during which any abnomial magnetic effects cuasing instability are likely to have declined.

If there is lack of stability between 52 and 53, one can of course immediately go back from 53 to normal current intensity at 61, without attempting to achieve the cell stability by a further step of the first current intensity reduction.

For each cell, a calculation is made of the concentration of A1 0 based on the extrapolated value of E and the known relationship between E and A1 0 concentration. Then the computer instructs an automatic crust breaker to draw back more or less: of the A1 0 at 23 before breaking the crust, so that a correspondingly smaller or larger addition of A1 0 is made to the cell when the automatic crust breaker next breaks the crust of that cell.

The cells connected in series are generally given normal servicing in succession. If there are as many current reduction operations per day as there are normal servicings per cell per day, then every cell is measured at least once between two normal servicings. Preferably there are twice as many current reductions per day as there are normal services per cell per day.

What is claimed is:

l. A method of controlling the supply of Al O by at least two combined voltage and current intensity or Amperage measurements during the operation of a cell for electrolytic recovery of aluminum, comprising the following operations carried out successively: (a) measuring the cell current intensity at a normal working point; (b) thereafter making a first reduction of the current intensity by at least 5 percent and at most 8 percent in one or more steps until attainment of cell stability and straightness of the voltage-current graph, the percentages being of the current intensity at the working point; (c) subsequently measuring the values of voltage and current intensity of the cell at a time period between one-half of one minute and 2 minutes after the first current intensity reduction, this constituting the first of said combined measurements; ((1) making thereafter, a second reduction of current'intensity to a total current intensity reduction of at most 25 percent, but at least of 10 percent more than the first reduction, up to at latest 10 minutes after the first current intensity reduction; (e) subsequently measuring the values of voltage and current intensity of the cell at a time period between onehalf of one minute and 2 minutes after the second current intensity reduction, this constituting the second combined. measurement; (f)

' thereafter restoring the current intensity to the normal current intensity at (e.g., 100,000 amps) and one working point; (g) extrapolating to the cell voltage E,, at cell current equalling zero from the values of voltage and current intensity after the first current intensity reduction and after the second current intensity reduction; (h) determining the concentration of A1 0 in the cell on the basis of the relationship between the term E of the EMF independent of current density and the A1 0 concentration: (i) supplying a larger or smaller addition of Al O to the electrolyte at each servicing of the cell, if the A1 0 concentration departs downwards or upwards from a predetermined value.

2. A method of determining the A1 0 concentration in an operating cell for electrolytic recovery of aluminum, for use in subsequently controlling the Al O concentration in the endeavor to maintain optimum conditions in the cell, by determining at [least two interpolation values, each interpolation value being defined by a measured potential difference across the cell corresponding to a selected current level, and by the measured value of said selected current level, thereafter interpolating the interpolation values to determine a value for the potential difference, E corresponding to a zero current level, the concentration of A1 0 being directly related to and therefore determinable from E said interpolation values being determined from the measurements comprising the steps:

a. determining the current level through the cell, at

a normal working value for the cell;

b. thereafter making a first reduction in current intensity by a minimum of approximately 5 percent to a maximum of approximately 8 percent of the normal working value, in one or more operations, until there is attained cell stability as determinable by the ratio of the voltage across the cell to the current through the cell, reaching a steady state value:

c. thereafter making a first measurement of the values of the potential difference across the cell and the corresponding current through the cell, said first measurement being made at a time of approximately one half of one minute to approximately two minutes measured from the time of said first reduction in current intensity, whereby the first of said interpolation values is determined; d. subsequently making a second reduction in current intensity through the cell, said second reduction being made at the latest ten minutes after said first reduction and being of an amount whereby said second reduction and said first reduction do not exceed a total reduction of approximately 25 percent in the current intensity from said normal working value but said second reduction being at least 10 percent greater than said first reduction;

. thereafter making a second measurement of the values of the potential difference across the cell and the corresponding current through the cell, said second measurement being made at a time of approximately one-half of one minute to approximately two minutes, from the time of said second reduction in current intensity, whereby the second interpolation value is obtained 

1. A METHOD OF CONTROLLING THE SUPPLY OF AL2O3 BY AT LEAST TWO COMBINED VOLTAGE AND CURRENT INTENSITY OR AMPERAGE MEASURMENTS DURING THE OPERATION OF A CELL FOR ELECTROLYTIC RECOVERY OF ALUMINUM, COMPRISING THE FOLLOWING OPERATIONS CARRIED OUT SUCCESSIVELY: (A) MEASURING THE CELL CURRENT INTENSITY AT A NORMAL WORKING POINT; (B) THEREAFTER MAKING A FIRST REDUCTION OF THE CURRENT INTENSITY BY AT LEAST 5 PERCENT AND AT MOST 8 PERCENT IN ONE OR MORE STEPS UNTIL ATTAINMENT OF CELL STABILITY AND STRAIGHTNESS OF THE VOLTAGE-CURRENT GRAPH, THE PERCENTAGES BEING OF THE CURRENT INTENSITY AT THE WORKING POINT; (C) SUBSEQUENTLY MEASURING THE VALUES OF VOLTAGE AND CURRENT INTENSITY OF THE CELL AT A TIME PERIOD BETWEEN ONE-HALF OF ONE MINUTE AND 2 MINUTES AFTER THE FIRST CURRENT INTENSITY REDUCTON, THIS CONSTITUTING THE FIRST OF SAID COMBINED MEASUREMENTS; (D) MAKING THEREAFTER, A SECOND REDUCTION OF CURRENT INTENSITY TO A TOTAL CURRENT INTENSITY REDUCTION OF AT MOST 25 PERCENT, BUT AT LEAST OF 10 PERCENT MORE THAN THE FIRST REDUCTION, UP TO AT LEAST 10 MINUTES AFTER THE FIRST CURRENT INTENSITY REDUCTION; (E) SUBSEQUENTLY MEASURING THE VALUES OF VOLTAGE AND CURRENT INTENSITY OF THE CELL AT A TIME PERIOD BETWEEN ONE-HALF OF ONE MINUTE AND 2 MINUTES AFTER THE SECOND CURRENT INTENSITY REDUCTION, THIS CONSTITUTING THE SECOND COMBINED MEASUREMENT: (F) THEREAFTER RESTORING THE CURRENT INTENSITY OF THE NORMAL WORKING POINT; (G) EXTRAPOLATING TO THE CELL VOLTAGE E* AT CELL CURRENT EQUALLING ZERO FRM THE VALUES OF VOLTAGE AND CURRENT INTENSITY AFTER THE FIRST CURRENT INTENSITY REDUCTION AND AFTER THE SECOND CURRENT INTENSITY REDUCTION; (H) DETERMINING THE CONCENTRATION OF AL2O3 IN THE CELL ON THE BASIS OF THE RELATIONSHIP BETWEEN THE TERM E$ OF THE EMF INDEPENDENT OF CURRENT DENSITY AND THE AL2O3 TO THE ELECTROLYTE PLYING A LARGER OR SMALLER ADDITION OF AL2O3 TO THE ELECTROLYTE AT EACH SERVICING OF THE CELL, IF THE AL2O3 CONCENTRATION DEPARTS DOWNWARDS OR UPWARDS FROM A PREDETERMINED VALUE.
 2. A method of determining the Al2O3 concentration in an operating cell for electrolytic recovery of aluminum, for use in subsequently controlling the Al2O3 concentration in the endeavor to maintain optimum conditions in the cell, by determining at least two interpolation values, each interpolation value being defined by a measured potential difference across the cell corresponding to a selected current level, and by the measured value of said selected current level, thereafter interpolating the interpolation values to determine a value for the potential difference, Eo corresponding to a zero current level, the concentration of Al2O3 being directly related to and therefore determinable from Eo, said interpolation values being determined from the measurements comprising the steps: a. determining the current level through the cell, at a normal working value for the cell; b. thereafter making a first reduction in current intensity by a minimum of approximately 5 percent to a maximum of approximately 8 percent of the normal working value, in one or more operations, until there is attained cell stability as determinable by the ratio of the voltage across the cell to the current through the cell, reaching a steady state value: c. thereafter making a first measurement of the values of the potential difference across the cell and the corresponding current through the cell, said first measurement being made at a time of approximately one half of one minute to approximately two minutes measured from the time of said first reduction in current intensity, whereby the first of said interpolation values is determined; d. subsequently making a second reduction in current intensity through the cell, said second reduction being made at the latest ten minutes after said first reduction and being of an amount whereby said second reduction and said first reduction do not exceed a total reduction of approximately 25 percent in the current intensity from said normal working value but said second reduction being at least 10 percent greater than said first reduction; e. thereafter making a second measurement of the values of the potential difference across the cell and the corresponding current through the cell, said second measurement being made at a time of approximately one-half of one minute to approximately two minutes, from the time of said second reduction in current intensity, whereby the second interpolation value is obtained. 